Eustacy means that for 84,000 years this ice stored all the precipitation that fell on it lowering the Ocean's level over 100 metres.

Isostacy means

that the weight of that ice warped the land's surfaces (i.e. it pushed the land down about 200 metres around central Ontario) when and while it covered it

and the same land slowly rebounded raised when the retreating ice cap uncovered it (this has been measured and is recorded.) Actually it is still rebounding around Central Ontario and nobody knows when it will stop!

Newton's law (equal and opposite reaction) applied. Down near Windsor/Detroit the land was raised and it has been sinking since the ice sheet retreated! That is why Lake Erie drained into Lake Huron and out the Mattawa/Ottawa valley from 7,600 to 4,700 years ago. After that the waters started to drain over Niagara and out the Saint Lawrence.